In the purest sense, a supercharger is any device that provides the engine with a greater charge of air (not fuel) than the cylinder can could intake through the down motion of the piston. It is a pump that specializes in pushing air into the intake, while the engine itself is not so specialized.

In practice there is a huge division: turbochargers are superchargers that use energy from the exhaust gases to drive a turbine which drives another turbine that pressurizes the intake. Basically every other device that does the above but isn't a turbo is called a supercharger.

Even more generally, when people say "supercharger" they are generally referring to a particular type of supercharger, called a roots-type supercharger. This type of supercharger uses two rotors with lobes that spin together pushing air into the intake. It is driven via a belt, and its spin is proportional to the speed of the engine's rpms, unlike a turbo which is more proportional exponent of the engine's speed (well, not really, but for our purposes, pretend...). This leads to a difference in the response of the roots-type supercharger and a turbo. A roots-type supercharger gives the engine a boost in the lower range, and raises peak torque (which is often used to express the low rpm power of the engine) but near the top end, the supercharger is inefficent. A turbo doesn't really give a large boost in the low range, because the turbo must spool, leading to the infamous turbo lag. However, once the turbo is spooled up, it is a much more efficent design, and raises the top end and horsepower signifcantly more than a supercharger.

A supercharger cannot reach the lofty heights of a turbo because when spun at high speed, the supercharger causes a huge amount of heat, a result of concussions as tiny packets of 1 bar (atmospheric pressure) air smack into a 1.75 bar mass of pressurized air. A supercharger doesn't have "internal compression," that is, it doesn't compress the air as it is running through the device. Hence, it is also called a blower. The high temperature of the charge resulting from a supercharger at high rpms is very very bad. It is the destroyer of engines, the ruiner of cars, and the sacker of races. Hot air increases your chances of detonation, or preignition.

Most people say that a supercharger will not go over 15psi (or approx 1 bar), this is probably true, but even at one bar, the supercharger is generating so much heat, it's probably doing more harm than good. It would be a good idea to use an intercooler or some other charge cooling device (or a combination) if you really want to run a supercharger at high boost.

One of the biggest manufacturers of roots-type superchargers is Eaton.

All the the previous, of course, only applies to roots-type superchargers. There are several other types (besides turbos, which are generally segregated): centrifugal, lysholm, and roots are probably the main types used today.

A centrifugal, also known as a Vortech (after a company who makes them) is about as different from a roots as it could get, and despite that people often call it a supercharger, suggesting that it is a roots type. A centrifugal uses a turbine mated to a belt drive or a electric motor drive. This gives you the downsides of a turbo and and the downsides of a roots all in one. First of all, it is driven proportionally to the engine's speed, so you will never get the exponential speed curve that an exhaust driven turbine affords you. Second, you are using a turbine (at either too high or too low of a speed) so all your gains are going to be biased to the top end (but you are limited because of the linear speed curve). Bosch suggests a continuously variable transmission to allow the turbine to spin at the appropriate speed. Why not just get a turbo? The best success I have seen with these is on very torquey truck engines, as they will give a little boost up top.

A lysholm, aka whipple or whipplecharger (again, after a company that makes them) is sort of a modified roots. It uses a set of screws that compress the air as the travel up the screws to make up for some of the inefficencies of roots-type superchargers. They are also easier to spin faster. They are very common in Australia and Europe, but rarely seen in America and Japan.

I notice a lot of people seem to be heavily biased against superchargers. There are plenty of writeups extolling the virtues of turbochargers. This writeup, then, will be in defense of the oft-maligned supercharger.

Positive-displacement superchargers, such as the twin-screw Eaton models, will usually make full boost by the time they're somewhere between 1,000 and 1,500RPM, and make constant boost all the way up to redline. This "breaks" at very low and very high RPMs due to volumetric efficiency. The fastest-spooling twin turbo systems (two small turbos rather than one large one; this is done to reduce turbo lag) are lucky to make full boost by 1,800RPM. To illustrate why this is a good thing, take a look at this chart.

In this example, a supercharger and turbocharger (or a centrifugal supercharger, which has a similar power curve) have been set for a maximum boost of 9PSI. For the supercharger, this is determined by pulley size: smaller pulleys = faster rotation = higher boost. For a turbocharger, this is accomplished by a wastegate. They both achieve the same maximum boost level. However, the turbocharger does NOTHING for low-end torque. If you are looking for a mod that will improve driveability, and you have a four-cylinder engine, and both supercharger and turbocharger kits are available for your engine, you're cheating yourself if you don't evaluate the supercharger. The reason? Four-bangers are notoriously lacking in torque, especially at low revs. Superchargers do a lot for engine torque down low when it's really needed. Turbos don't.

The big argument a turbo snob will use is that turbos use "wasted" power from an engine, whereas superchargers have to tap the crank for their motive force. What they don't mention is that turbochargers raise the backpressure inside the cylinders. A *little* backpressure is good, as it helps with the scavenging effect that helps suck exhaust out of the tailpipe, but turbochargers contribute more than is actually helpful. The hot wheelturbine of a turbocharger slows down the exhaust gases (you didn't think kinetic energy was free, did you?), meaning that the engine has to work all the harder to complete its breathing cycle.

What they should really say is, "I like how it sounds when it spools up, and I like the way the blow-off valve sounds when I shift." Superchargers are usually a bit quieter than turbochargers, and don't usually need a blow-off valve. Instead, they use a bypass valve to zero out boost when the engine is at cruise or idle.

Turbo snobs often say, "Turbos make more peak power than superchargers." Refer to the chart above. What good is that if your car is still gutless below 3,000RPM? This argument is also silly because you can tune an Eaton supercharger to run up to 30PSI. Parasitic power loss in an Eaton supercharger is usually around one horsepower. BFD.

Speaking of 30PSI, that's what the top-fuel dragsters run. And it comes from a supercharger, not a turbocharger. Know why? Top-fuel dragsters can't afford to have extra-restrictive exhaust systems.

Detonation, or pre-ignition, can be a problem with both superchargers and turbochargers. The solution is to either make a blower that doesn't add as much heat (like the twin-screw Eaton models), or use an intercooler. It adds some cost, but it is well-proven. Another way to handle detonation is to use higher-octane gas, but that may require that the spark timing be advanced.

Last, superchargers don't have to be "warmed up" or "cooled down," and many of them feature self-contained lubrication systems with oil that doesn't have to be changed for up to 100,000 miles of use. Turbochargers generally have to share oil with the engine, and many of them will die prematurely if they aren't given some time to warm up (engine exhaust gas is HOT!) and some time to cool down (rapid contraction is hard on the components).

Is the forth album from the fantastic band Machine Head, the others being Burn my Eyes, The More Things Change and The Burning Red. This Album is quite different from their previous ones, being more focused on the vocals, and less on the raw power of the drums and guitar. This may not appeal to fans of Machine Heads other work strait away, but be warned, this album grows on you. I highly recommend this album for any fan of heavy music. Here are the Tracks